1. Field of the Invention
The present invention is utilized in a mobile radio communication system and, particularly, the present invention is utilized in a receiving circuit of a mobile terminal. The present invention relates to a receiver of a CDMA (Code Division Multiple Access) system. The present invention relates to an improvement of the diversity hand-over system in which a mobile terminal, which receives signals from a plurality of base stations, executes a hand-over from one of the base stations from which the mobile terminal is receiving a signal to another base station, while keeping the signal reception from the one base station. The present invention also relates to a novel control logic for assigning a number of detected receiving signal branches to a limited number of fingers. The present invention relates to an improvement of a rake receiving circuit, which is inputted with a number of branch signals, such that the rake receiving circuit can stably continue a signal receiving with minimum possibility of step-out when a hand-over is executed.
2. Description of Related Art
In order to perform a hand-over when a mobile terminal moves from one of a plurality of base stations to another, it is necessary to measure delay profiles of a plurality (3 at most) of cells or sectors, that is, branch signals (BH) to detect multiple paths. In a case where a diversity hand-over is performed with using a limited number of fingers in order to reduce the size of hardware, the fingers are distributed to the respective branch signals according to a system to be described below.
The system may be the fixed assignment system shown in FIG. 6, which shows a block diagram of the fixed assignment system schematically. In a hand-over, branch signals are assigned to fixed fingers, respectively. When a plurality (3 in this case) of branch signals are activated as shown in FIG. 6, a path searching for one branch signal is performed three times independently. Paths selected by the respective branch signals are assigned to the respective fingers according to a predetermined assigning pattern as shown in FIG. 7.
Another system may be the adaptive assignment system and FIG. 8 is a block diagram showing a construction of the adaptive assignment system schematically. Paths detected by the respective branch signals are assigned to fingers in order of level of the receiving signal or level of SNR (Signal to Noise Ratio) of receiving signal That is, after a peak detection is performed for each branch signal, the detected peaks of the branch signals are sorted in order of the level of the peaks or in order of level of SNR of the branch signals. Synchronization is determined on the basis of an information of the peaks of the branch signals after sorted. Therefore, the number of fingers to be assigned to each of the branch signals depends upon the state of paths.
Merits and demerits of the fixed assignment system will be described below.
[Merit] There is no need of switching the branch signals received by the respective fingers except a case where a branch signal is added or deleted.
[Demerit] There is a possibility of instantaneous cutoff of branch signals since a finger used is switched when a branch signal is added or deleted.
Merits and demerits of the adaptive assignment system will be described below.
[Merit] The effect of combining may be maximal since paths are assigned in order of SNR. Further, there is no instantaneous cutoff of branch signal when a branch is added or deleted.
[Demerit] Since the number of fingers assigned to a branch signal whose level or SNR is relatively low may be at least reduced, there is a possibility of step-off of the branch signal. Since, in such case, the sequence continuously requests an addition of a branch signal, there may be a flutter in addition or deletion of branch signal, so that the hand-over operation becomes unstable.
In view of the merits and demerits of the fixed assignment system and the adaptive assignment system, it has been requested to develop a novel system capable of providing the merits of these systems while removing the demerits thereof.
The present invention was made in view of the background mentioned above and an object of the present invention is to provide a CDMA receiver, which has no instantaneous cutoff of branch signal at a time of addition or deletion of branch signal, operates stably at a time of hand-over, has high utilization efficiency of finger and can improve a signal receiving characteristics.
Another object of the present invention is to provide a CDMA receiving method.
According to the present invention, when a new branch signal to be added generated under condition that the number of effective paths of an existing branch signal is larger than the number of fingers assigned to the existing branch signal and the number of fingers assigned to all branch signals is smaller than an aimed number of fingers, the new branch signal is assigned to an unused finger if any and, if there is no unused finger, an ineffective finger caused due to such reason as low receiving quality is switched to a finger for the new branch signal. The aimed number of fingers means the number of fingers per branch signal when the fingers are assigned equally to all of branch signals.
In the case where there is no unused finger, the numbers of fingers assigned to a plurality of other branch signals are compared with each other and, when the number of fingers assigned to one of the other branch signals, which is assigned to the largest number of fingers, is larger than the aimed number of fingers, a finger of fingers assigned to the one branch signal, which is lowest in level, is switched for a finger for the one branch signal.
That is, when the number of fingers assigned to the one branch signal is not larger than the aimed number and there is a path to be added, a finger to be replaced is searched for an unused finger first and for a finger among at least the aimed number of fingers assigned to a branch signal, which is in the lowest level, second.
In a case where the aimed number of fingers is satisfied and there is a path to be added, an unused finger is used if any. When there is only effective fingers, the number of fingers of the branch signal is not increased.
Since, according to such algorithm, there is no need of switching the branch signal received by the respective fingers every time a branch signal is added or deleted, there is no instantaneous cut-off and the hand-over operation can be stabilized. Further, by assigning an unused finger with highest priority, it is possible to improve the efficiency of finger use to thereby improve the signal receiving characteristics.
That is, according to a first aspect of the present invention, a CDMA receiver comprises an AD converter for converting analog receiving signals from a plurality of base stations into digital signals and outputting a plurality of branch signals to be transmitted through different propagation paths, a path searcher for assigning the branch signals to fingers according to delay profiles thereof, a plurality of finger portions to be assigned to the branch signals according to a finger assignment performed in the path searcher and a rake combiner for performing a maximum ratio combining of powers of a plurality of mutually delayed signals obtained from the fingers.
The CDMA receiver is featured by that the path searcher comprises a plurality of peak detectors provided correspondingly to the respective branch signals for detecting correlation peaks of the respective branch signals from the delay profiles of the branch signals and producing an information of peaks of the branch signals, respectively, a plurality of finger assignors provided correspondingly to the respective branch signals for assigning the branch signals to the finger portions according to the peak information and outputting path timings of the plurality of the finger portions and a finger distributor for setting an aimed number of fingers to be assigned to each of the finger portions according to the path timings such that the number of fingers assigned to each of the plurality of the finger portions becomes even for the plurality of the branch signals and feeding the numbers of fingers assigned to the respective finger portions, branch numbers of the fingers and the timing information back to the finger assignors.
The finger distributor preferably comprises means for assigning a branch signal to an unused finger or an ineffective finger having low receiving quality, when the branch signal is added.
According to a second aspect of the present invention, a CDMA receiving method includes the steps of converting analog receiving signals from a plurality of base stations into digital signals, outputting a plurality of branch signals to be transmitted through different propagation paths, assigning a finger to each of the branch signals according to delay profiles thereof, assigning the plurality of the branch signals to a plurality of fingers according to the finger assignment performed by the path searcher and performing a maximum ratio combining of powers of a plurality of mutually delayed signals obtained from the fingers.
This CDMA receiving method is featured by comprising the steps of detecting correlation peaks of the plurality of the branch signals from the delay profiles of the branch signals and producing an information of peaks of the branch signals, respectively, assigning the plurality of the fingers to the plurality of the branch signals according to the peak information and outputting path timings of the plurality of the fingers, setting an aimed number of fingers according to the path timings such that the number of fingers assigned to each of the plurality of the fingers is even for the plurality of the branch signals and feeding the numbers of fingers assigned to the respective fingers, branch numbers of the fingers and the timing information back to the finger assignors.
It is preferable that a branch signal is assigned to an unused finger or an ineffective finger having low signal receiving quality at a time when the branch signal is added.